SO ₂ emissions and lifetimes derived from TROPOMI observations over India using a flux-divergence method

The rapid development of the economy and the implementation of environmental policies adapted in India has led to fast changes of regional SO ₂ emissions. We present a monthly SO ₂ emission inventory for India covering December 2018 to November 2023 based on the TROPOMI Level-2 COBRA SO ₂ dataset, by using an improved flux-divergence method and estimated local SO ₂ lifetime which includes both its chemical loss and dry deposition. We update the methodology to use the daily CAMS model output estimates of the hydroxyl-radical distribution as well as the measured dry deposition velocity to account for the variability in the tropospheric SO ₂ lifetime. The results show the application of the local SO ₂ lifetime improves the accuracy of SO ₂ emissions estimation when compared to calculations using a constant lifetime. Our improved flux-divergence method reduced the spreading of the point source emissions compared to the standard flux-divergence method. The averaged SO ₂ emissions covering the recent 5 years are about 5.2 Tg year ^-1, which is lower than the bottom-up emissions of 11.0 Tg year ^-1 from CAMS-GLOB-ANT v5.3. The total emissions from the 92 largest point source emissions are estimated to be 2.9 Tg year ^-1, lower than the estimation of 5.2 Tg year ^-1 from the global SO ₂ catalog MSAQSO ₂LV4. We argue that for other important regions that have high SO ₂ emissions, the variability in the SO ₂ lifetime becomes more important to account for estimating top-down SO ₂ emissions.